Thermal plasma processes use electricity as an energy source. For example, a gas is passed through an electric arc struck between an anode and a cathode or through a coil to which high frequency current is applied. This electrical energy is converted to thermal energy, as the gas is heated when it passes through the arc discharge or through the inductive coupling. The heated gas is useful as an energy carrier for a subsequent chemical reaction. Among the advantages of using electricity as a thermal energy source is that extremely high temperatures (higher than normal combustion temperatures) can be obtained in the gas and combustion products can be avoided. Devices that transfer electrical energy to thermal energy in this manner are sometimes called plasma heaters. Plasma heaters are available that provide gas temperatures up to about 20,000.degree. K. Gas in the heater is in a partially ionized plasma state of being electrically conductive but still retaining electrical neutrality. This plasma state lasts for a short distance downstream of the heater exit before the electrons and ions recombine as the hot gasses cool. Among commercially available heaters are inductively coupled plasma torches (providing gas temperatures up to about 10,000.degree. K.), welding torches, and the like.
A variety of plasma based processes are known or are in various stages of development. Thus, for example, Herman, Scientific American. pp. 112-117 (September 1988) describes plasma-sprayed coatings of ceramics, metals and polymers in air or in vacuum. Vukanovic et al., U.S. Pat. No. 4,505,947, issued Mar. 19, 1985 describes coating methods utilizing an arc plasma described as a non-local thermal equilibrium plasma said to be useful for depositing silicon and other semiconductor materials onto substrates.
Plasma processes may be generally viewed as either thermal, or high pressure, processes or cold, low pressure, processes. Low pressure processes are operated at sub-atmospheric pressures, typically in the range of about 1 Torr. For example, the etching of semiconductor material in VLSI circuit manufacture is by means of a cold plasma process. However, low pressure processes, when used to deposit a reaction product, tend to have relatively low deposition rates.
A typical plasma processing system contains a power supply (either AC or DC), a plasma heater (such as a torch), a reactor downstream of the heater, a plasma gas delivery system, a reactant feed system, a product collection system, a coolant system, and a control system.
Plasma processing has been suggested for the production of silicon by the reduction of SiCl.sub.4, Coudert et al., Plasma Processing and Plasma Chemistry, Volume 2, No. 4, pp. 399-419 (1982). At sufficiently high temperatures, the chlorine atoms will be removed from the silicon tetrachloride molecule and recombined with hydrogen to form hydrochloric acid.
Another use of plasma has been to deposit diamond from a hydrogen and hydrocarbon mixture gas. U.S. Pat. No. 4,434,188, issued Feb. 28, 1984, inventors Tamo et al., describes a method for synthesizing diamond where a plasma is generated at a pressure from 0.05 to 400 Torr by microwave discharge. The plasma is formed from a mixture of hydrocarbon and hydrogen gasses carried by an unactive gas such as argon. U.S. Pat. No. 4,717,622, issued Jan. 5, 1988, inventors Purokawa et al. describes a plasma process conducted at low pressure (10-100 Pa, or 7.5-75 Torr) in which a plasma created by exciting a mixture of hydrocarbon and argon gasses is blown onto a substrate, such as a magnetic recording medium, to deposit a high hardness carbon. In a report of symposium proceedings held Aug. 31 through Sept. 4, 1987 was described the preparation of microcrystals and microcrystalline films of diamond on molybdenum substrates in a rf induction thermal plasma by using an argon-hydrogen-methane mixture under 1 atmosphere pressure. ISPC/TOKYO 1987, Vol. 4, Matsumoto et al., pp. no. S7-03.
However, the deposition rates of the known plasma processes involving dissociation, or decomposition, of a multiatomic reactant have tended to be disappointing and the quality of films deposited is not always satisfactory.